Japanese Patent Application Publication No. 2001-17470 discloses an absorbing core having a tissue-like hydrophilic sheet bonded to the liquid-holding material to prevent deformation of the absorbing core. This absorbing core, however, has insufficient strength in a wet state after urination.
Japanese Patent Application Publication No. 2002-512082 discusses an absorbent article in which the entire liquid-holding material is covered with a hydrophilic non-woven cloth and is bonded to a surface sheet to prevent deformation and wetback. This technique, however, does not reduce permeation of urine from the inside to the outside of the liquid-holding material because the non-woven cloth remains hydrophilic after urination multiple times. Therefore, wetback from the liquid-holding material is not sufficiently prevented when the pressure of the body is applied thereto. Although wetback can be reduced by using an insufficiently hydrophilic non-woven cloth, it is readily conceivable that such a non-woven cloth would hinder normal absorption of urine.
Japanese Patent Application Publication No. 9-56748 discusses absorbent article in which a hydrophilic synthetic fiber layer and a sheet containing cotton pulp are disposed in layers as a second layer between a top sheet and an absorbing core to reduce wetback and urine remaining on the top sheet. Japanese Patent Application Publication No. 2009-148322 discusses an absorbent article including low-diffusion filter paper for improving the absorption rate.
Providing a second sheet is effective in improving the absorption rate and reducing wetback to a certain degree; however, the structure of the absorbing core is not disclosed in these documents. Assuming that an existing absorbing core covered in its entirety with a hydrophilic sheet such as a non-woven cloth is used, it is difficult to simultaneously reduce wetback and improve the absorption rate, as in the technique discussed in Patent Literature 2 above.